1. Field of the Invention
The invention relates generally to telecommunication systems and more particularly to development of such systems to allow use of different communication formats.
2. Description of the Related Art
The most common communication format for telecommunication systems is time division multiplexing (TDM) which is a continuous bit rate service concatenated as multiples of 64 kb/s channels. The TDM technique is particularly effective for voice communication because it is connection-oriented, i.e. a connection link is established initially and is maintained throughout a communication session. Also, the speed and reliability are sufficient for voice.
However, TDM is not particularly effective for data traffic because it is inflexible and does not accommodate the “bursty” nature of data very well. It was for this reason that packet-based techniques have been developed for data communication. For example, the Internet Protocol (IP) technique is used for much Internet service provider (ISP) and local area network (LAN) communication. This technique uses variable-length packets, each of which is independently routed. This means that the technique is not connection-oriented and the bandwidth for a call is not guaranteed for the duration of the call. The general view is that voice subscribers would not tolerate the ensuing unpredictability of call quality. Also, the equipment required for processing to achieve the required speed for voice is quite complex. Therefore, while the IP technique is an approach which may be used to some extent in the future for both voice and data telecommunication networks, problems need to be overcome.
Another packet-based technique is the asynchronous transfer mode (ATM) technique. This has the advantages that it is packet-based and so can handle bursty data traffic well, and is also connection-oriented. In this technique, data is transmitted in fixed-length packets which are typically 53 Bytes in length, having a 48 Byte payload and a 5 Byte header. The header comprises a virtual path identifier (VPI) and a virtual channel identifier (VCI). According to the ATM standard, the cell transfer technique is common to all services and therefore a single ATM stream of cells can include a wide range of different services. Because the cells have a fixed length, timing and control is more predictable. On the other hand, packets in the IP technique are often much longer than ATM cells.
Packet-based techniques are radically different from the TDM technique and therefore it is not practical for a telecommunication operator to change over in a short period of time. In order to introduce packet-based techniques such as ATM, adaptation processes have been developed which are service-specific as different reconstruction techniques are involved. This has led to a large degree of complexity and expense.
It is also known to provide a hybrid system in which TDM and ATM signals are combined in a frame for communication over a link. Such an arrangement is described in PCT Patent Specification No. WO 97/18649 (DSC Communications Corp.). While this arrangement is suitable for specific application areas such as provision of services to a home, it does not appear to be efficient enough for telecommunication networks. This is particularly the case if a number of different services are involved.
European Patent Specification No. EP614324 (Nippon) describes a system for separate TDM/ATM and ATM/TDM conversion. However, this system appears to be quite complex and to be difficult to scale up for increased transaction volume capacity.